1. Field of the Invention
The present invention relates to a knocking detecting device for an internal combustion engine which detects knocking in which abnormal combustion is occurred in the internal combustion engine, and more particularly to an improvement in the knocking detection precision for further improving the operation efficiency of the internal combustion engine.
2. Description of the Related Art
In the automobile industry, particularly in a field that uses the internal combustion engine, demands from users for high power engines are increasing, whereas there are demands for exhaust (emission) gas reduction from environment viewpoints. Expectation to develop a technique capable of satisfying both demands is growing rapidly in recent years.
In order to satisfy those demands, various technologies have been developed, in which there are tendencies that the combustion efficiency is enhanced to the maximum, to thereby provide a common solution to both conflicting requirements.
However, the possibility that the abnormal combustion which is called “knocking (knock)” occurs becomes higher as the combustion efficiency is made higher. The knocking is considered as an abnormal self ignition phenomenon that occurs in cylinders of the engine, and the occurrence of the knocking may cause the engine to be damaged, which is to be avoided.
Accordingly, in order to enhance the combustion efficiency, a technology to avoid the knocking is required, and a development of a knocking detection technology with higher precision has been required.
Up to now, a large number of knocking detecting methods have been proposed, and as a representative method, there is proposed a device for detecting knocking using vibration frequency of the engine (for example, refer to Japanese Patent No. 2684611). The knocking detecting device detects resonance frequencies within two or more cylinders to obtain a knocking indicator based on the sum of those resonance frequency components.
However, some resonance frequencies occur without attributing to the occurrence of knocking. A representative example thereof is a “ρ10 mode resonance frequency component,” which is described in Japanese Patent No. 2684611.
In a case where the degree of disturbance within a combustion chamber which is induced by knocking is defined as the intensity of knocking, the magnitude of the ρ10 mode resonance frequency component can be said as precisely expressing the knocking intensity. On the contrary, there arises such a problem that the ρ10 mode resonance frequency component is the lowest resonance vibration mode within the engine combustion chamber, and readily occurs by merely giving a small energy.
That is, the vibration is caused by not knocking but some disturbance of combustion, and it is impossible to discriminate whether knocking occurs, or not, according to the resonance vibration. For example, there has been known that the ρ10 mode resonance frequency component frequently occurs in an excessive air-fuel ratio rich state or at the time of ignition at an excessive spark delay position. Also, there are many examples in which such a ρ10 mode resonance frequency component always occurs.
From the above-mentioned viewpoints, there arises such a problem that it is difficult to separate the ρ10 mode resonance frequency components from noises in the knocking indicator taking the sum including the ρ10 mode resonance frequency components.
On the contrary, a large energy is required to generate higher-order resonance vibrations than the ρ10 mode resonance frequency components within the combustion chamber. Therefore, because the higher-order resonance vibrations do not easily occur, there has been known that the higher-order resonance vibrations precisely indicate whether knocking occurs, or not. As a result, there are many knocking detecting devices that mainly use the higher-order resonance vibrations.
However, because the frequencies of the higher-order resonance vibrations are close to the resonance vibration frequencies of an engine block, those knocking detecting devices suffer from a large number of problems. That is, it is difficult to separate the higher-order resonance vibrations from the resonance vibration frequency of the engine block, and from impulse noises, and the attenuation of a signal is quick because of unstable resonance vibrations. Also, the quantity of frequency shift due to a change in temperature or state within the combustion chamber is large, and it is difficult to ensure the signal level. Those problems induce barriers for achieving the required knocking detection precision.